1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive actuator and a method for fabricating the same, and more particularly to a piezoelectric/electrostrictive actuator including a vibrating plate made of metal, organic compound or ceramic, and a piezoelectric element formed by forming a piezoelectric/electrostrictive film, in the form of a thin or thick film, on the vibrating plate using specific ceramic powder, and thermally treating the piezoelectric/electrostrictive film at a relatively low temperature, thereby causing the piezoelectric/electrostrictive film to be integral with the vibrating plate, so that the fabrication of the piezoelectric element can be achieved without using a third material for bonding the piezoelectric element to the vibrating plate while minimizing a degradation in the properties of the vibrating plate and being capable of patterning the piezoelectric element during the formation of the piezoelectric element, thereby enabling a simultaneous fabrication of a plurality of piezoelectric/electrostrictive actuators without any requirement of post treatments. The present invention also relates to a method for fabricating such a piezoelectric/electrostrictive actuator.
2. Description of the Prior Art
Generally, actuators include a vibrating plate, a lower electrode formed on the vibrating plate, a piezoelectric/electrostrictive film formed on the lower electrode, and an upper electrode formed over the piezoelectric/electrostrictive film.
In such actuators having the above mentioned configuration, the piezoelectric/electrostrictive film arranged between the upper and lower electrodes deforms repeatedly as voltage is intermittently applied between the upper and low electrodes, so that it vibrates.
Conventionally, ceramic powder produced by solid state reaction method has been used to form piezoelectric/electrostrictive films for piezoelectric elements in piezoelectric/electrostrictive actuators.
The solid state reaction method, which is also called a xe2x80x9cmixed oxide methodxe2x80x9d, is a method for producing ceramic powder using an oxide or molten salt as a raw material. In such a solid state reaction method, raw materials in the form of powder are mixed together and then subjected to a thermal treatment at a temperature of 1,000 to 1,200xc2x0 C. The resulting mixture is milled and then sintered, thereby producing ceramic powder.
Typically, ceramic powder produced by the solid state reaction method has a relatively large grain size of 0.2 to 2 xcexcm, even though depending on the grain size of raw materials used. For this reason, the solid state reaction method is unsuitable to obtain a grain size of 0.1 xcexcm. Furthermore, the solid state reaction method has a drawback in that it involves a heat treatment at a high temperature of 1,000xc2x0 C. or more.
In the fabrication of various film devices using ceramics, a method has conventionally been used in which ceramic paste is produced from ceramic powder, and then subjected to a thermal treatment in a state printed or molded on a vibrating plate.
In conventional ceramic paste preparation methods, a mixture is prepared which consists of a binder, a vehicle, a plasticizer, and a dispersant. The mixture is dissolved in a solvent, thereby producing a solution. Ceramic grains, which are prepared by a solid state reaction method to have a mean grain size of 1 xcexcm, are then added to the solution. The resulting mixture is then stirred.
In order to fabricate a piezoelectric/electrostrictive film, the ceramic paste prepared by the above mentioned method is printed on a vibrating plate, dried at 130xc2x0 C., and then subjected to a heat treatment at 1,000xc2x0 C. or more. However, this method involves a problem in that an additional thermal treatment at a temperature of 500xc2x0 C. or more should be carried out after the drying process in order to achieve a binder removal for completely removing the organic ingredients added prior to the thermal treatment.
The ceramic paste, which is produced by this method, cannot be formed into a desired shape at a low temperature because it contains ceramic grains of a large size. For this reason, the thermal treatment is carried out at a temperature of 1,000xc2x0 C. However, this results in a limited selection of usable vibrating plates.
Conventional methods for fabricating piezoelectric/electrostrictive actuators are mainly classified into those associated with a vibrating plate made of metal or organic compound, and those associated with a vibrating plate made of ceramic.
First, conventional methods associated with a vibrating plate made of metal or organic compound will be described.
An example of a method for fabricating a piezoelectric/electrostrictive actuator, which is associated with a vibrating plate made of an insulating organic compound of resin, is disclosed in Japanese Patent Laid-open Publication No. Heisei 9-300609.
Where a vibrating plate made of metal or organic compound is used, a degradation in the properties thereof occurs inevitably due to a high temperature applied thereto. To this end, a piezoelectric plate, which is to be formed on the vibrating plate, is formed or fabricated separately from the vibrating plate. The piezoelectric plate is then bonded to the vibrating plate by means of a third material. The resulting structure is then mechanically cut into a desired size, so as to obtain a desired number of piezoelectric/electrostrictive actuators. This process is illustrated in FIG. 1.
This method is advantageous in that there is no requirement to use any process involving difficulties because the vibrating plate is made of resin or metal.
However, there is a problem caused by the use of the third material as a binder to bond the vibrating plate and piezoelectric element together. A binder layer having a non-uniform thickness may be formed in the binding process. Air bubbles may also exist in the binder layer. As a result, it is difficult to form a reliable binder layer.
In order to obtain a desired number of piezoelectric/electrostrictive actuators, the piezoelectric plate is sliced by a mechanical machining process. However, there is a limitation in the integration degree of piezoelectric/electrostrictive actuators due to a limitation on the mechanical machining process. As a result, it is difficult to obtain products exhibiting a superior quality and a high productivity.
Where a vibrating plate made of metal is used, an electric discharge phenomenon may occur because the distance between the upper or lower electrode and the vibrating plate is very short, namely, about 30 xcexcm.
Next, conventional methods associated with a vibrating plate made of ceramic will be described.
An example of a method for fabricating a piezoelectric/electrostrictive actuator, which is associated with a vibrating plate made of ceramic, is disclosed in U.S. Pat. No. 5,430,344. Where a thin or thick film for a piezoelectric element is directly formed over a vibrating plate, a degradation in the properties of the vibrating plate may occur during a sintering process carried out for the film at a high temperature. To this end, ceramic, which can withstand the sintering temperature for the piezoelectric element, is used to form a vibrating plate.
Where such ceramic vibrating plate is used, a lower electrode is formed on the ceramic vibrating plate using a thermal treatment at a temperature of 1,200xc2x0 C. or more. A piezoelectric element is then formed over the lower electrode using a thermal treatment at a temperature of 1,000xc2x0 C. or more. Finally, an upper electrode is formed over the piezoelectric element at a temperature of about 800xc2x0 C. This process is illustrated in FIG. 2.
A representative ceramic material used for the vibrating plate is a stabilized zirconia (ZrO2). Pure zirconia, which does not contain any addition ingredient, cannot be used for the vibrating plate because it involves a self collapse phenomenon occurring in the vicinity of 1,000xc2x0 C.
Although zirconia exhibits most superior mechanical properties in that no oxidation or variation thereof occurs even at a high temperature of 1,200xc2x0 C., there is a difficulty in shaping.
A vibrating plate made of zirconia is fabricated using a tape casting process. In the tape casting process, a blade is arranged over a film in such a fashion that a gap corresponding to a desired thickness of a zirconia film, to be formed, is defined between the blade and the film. A zirconia slurry is then coated to a desired thickness over the film while moving the film, thereby forming a zirconia film. The resulting structure is then cured at room temperature. Thereafter, the zirconia film is removed from the structure, thereby obtaining a vibrating plate having a desired thickness.
This method is advantageous in that it is possible to fabrication ultra-thin film type ceramic piezoelectric/electrostrictive actuators using a high temperature integral formation process such as ceramic micro formation process.
However, this method requires a structural shaping process, taking into consideration the shrinkage of the vibrating plate and piezoelectric element caused by the integral formation of those vibrating plate and piezoelectric element at a high temperature. Furthermore, since the ceramic vibrating plate has a very small thickness of, for example, 10 xcexcm, it is very difficult to maintain a desired quality during the formation of a green sheet, and to control a deformation of the vibrating plate during the high temperature formation.
The materials of elements constituting the piezoelectric actuator including the lower electrode material act chemically with one another, so that they may be degraded in properties. To this end, fabrication techniques involving difficulties should be used. For this reason, a degradation in yield occurs.
Therefore, an object of the invention is to solve the above mentioned problems and to provide a piezoelectric/electrostrictive actuator including a piezoelectric element formed using specific ceramic powder produced by a novel method, so that it is fabricated even at a low temperature, thereby being capable of using a vibrating plate made of various materials, directly forming a piezoelectric/electrostrictive film, as the piezoelectric element, on the vibrating plate without using any bonding process, and thus, achieving an improvement in quality, productivity, and yield, and to provide the piezoelectric/electrostrictive actuator.
In one aspect, the present invention provides a method for a piezoelectric/electrostrictive actuator comprising the steps of: preparing a vibrating plate made of metal; preparing ceramic oxide powder having a grain size of 5 xcexcm or less and containing lead (Pb) and titanium (Ti) as basic elements thereof, said ceramic oxide powder being produced by a non-explosive oxidative-reductive combustion reaction carried out at a temperature of 100 to 500xc2x0 C., preparing an organic solvent or water-based ceramic sol solution containing constituent ceramic elements identical or similar to those of said ceramic oxide powder, and mixing said ceramic oxide powder with said ceramic sol solution, thereby producing ceramic paste; forming a piezoelectric/electrostrictive film on said vibrating plate using said ceramic paste; thermally treating the resulting structure obtained after the formation of said piezoelectric/electrostrictive film at a temperature of 100 to 600xc2x0 C., thereby baking said piezoelectric/electrostrictive film; and forming an upper electrode on said piezoelectric/electrostrictive film.
In another aspect, the present invention provides a piezoelectric/electrostrictive actuator comprising: a vibrating plate made of metal; a piezoelectric/electrostrictive film formed on an upper surface of said vibrating plate by use of ceramic paste produced by preparing ceramic oxide powder having a grain size of 5 xcexcm or less and containing lead (Pb) and titanium (Ti) as basic elements thereof, said ceramic oxide powder being produced by a non-explosive oxidative-reductive combustion reaction carried out at a temperature of 100 to 500xc2x0 C., preparing an organic solvent or water-based ceramic sol solution containing constituent ceramic elements identical or similar to those of said ceramic oxide powder, and mixing said ceramic oxide powder with said ceramic sol solution; and an upper electrode coupled to an upper surface of said piezoelectric/electrostrictive film.
In another aspect, the present invention provides a method for a piezoelectric/electrostrictive actuator comprising the steps of: preparing a vibrating plate; forming a lower electrode on said vibrating plate; preparing ceramic oxide powder having a grain size of 5 xcexcm or less and containing lead (Pb) and titanium (Ti) as basic elements thereof, said ceramic oxide powder being produced by a non-explosive oxidative-reductive combustion reaction carried out at a temperature of 100 to 500xc2x0 C., preparing an organic solvent or water-based ceramic sol solution containing constituent ceramic elements identical or similar to those of said ceramic oxide powder, and mixing said ceramic oxide powder with said ceramic sol solution, thereby producing ceramic paste; forming a piezoelectric/electrostrictive film on said lower electrode using said ceramic paste; thermally treating the resulting structure obtained after the formation of said piezoelectric/electrostrictive film at a temperature of 100 to 600xc2x0 C., thereby baking said piezoelectric/electrostrictive film; and forming an upper electrode on said piezoelectric/electrostrictive film.
In another aspect, the present invention provides a piezoelectric/electrostrictive actuator comprising: a vibrating plate; a lower electrode coupled to an upper surface of said vibrating plate; a piezoelectric/electrostrictive film formed on an upper surface of said lower electrode by use of ceramic paste produced by preparing ceramic oxide powder having a grain size of 5 xcexcm or less and containing lead (Pb) and titanium (Ti) as basic elements thereof, said ceramic oxide powder being produced by a non-explosive oxidative-reductive combustion reaction carried out at a temperature of 100 to 500xc2x0 C., preparing an organic solvent or water-based ceramic sol solution containing constituent ceramic elements identical or similar to those of said ceramic oxide powder, and mixing said ceramic oxide powder with said ceramic sol solution; and an upper electrode coupled to an upper surface of said piezoelectric/electrostrictive film.